62 Inside the Living Cell 



very low, so that it is better to express the result directly in terms of 

 the number of mutations observed in the whole population. It was 

 found that the average frequency of a particular kind of mutation was 

 of the order of 25 X 10~^ per roentgen. This means that if one hun- 

 dred million mice received one roentgen each, twenty-five mutations 

 of this particular type would occur. The figure for the fruit fly is about 

 one tenth as much. 



We can apply this figure to human beings on the assumption that 

 the human mutation rate is the same as that of mice. Many of the 

 survivors of Hiroshima must have received 200 roentgens of radia- 

 tion. The chance of mutation of one particular gene in one of these 

 individuals may thus be expected to be about 200 X 25 X 10"^ i.e. 

 5,000 X 10-' or 0-5 in 10,000. As each individual probably has at 

 least 10,000 genes in all, which are equally liable to be affected, it fol- 

 lows that there is a high probability that each individual who survived 

 Hiroshima and received 200 roentgens of radiation carries at least 

 one mutated gene of one kind or another in the chromosomes of each 

 of his germ cells. 



We can now ask how ionizing radiations bring about mutations. It 

 is easy to observe that exposure to these radiations has a marked 

 effect on chromosomes. When the chromosomes of cells are 

 examined under the microscope after exposure to radiations, visible 

 effects are frequently found, such as breakage of a chromosome into 

 two portions (Plate 9). This is sometimes followed by more complex 

 happenings. If two chromosomes are broken, the two sets of broken 

 ends may join up with the wrong partners. When replication of the 

 chromosomes occurs, it often happens that the two new daughter 

 chromosomes do not separate from each other completely, but a 

 'bridge' forms between them which continues to hold them together. 

 Such effects are observed in a proportion of the cells, even after fairly 

 small doses of radiations. With plant material, Swanson observed at 

 least one chromosome aberration per cell with 150 roentgens of 

 radiation. 



An important feature of the genetic damage due to radiations is 

 that it is cumulative. It is believed that the effect is the same whether 

 a dose is delivered all at one time or in very small quantities over a 

 considerable period. It follows that exposure to very weak sources 

 of radiation for a long period may be as important as a large intensity 

 for a short time. 



It is therefore of great interest to estimate the prospects of genetic 

 damage to human beings from natural and other sources of radiation. 

 We are in fact all the time exposed to radiations from (1) cosmic rays; 

 (2) radioactive gases or dust in the ground and atmosphere produced 



